1. Field of the Invention 
This invention relates to improvements in controllers for the electric motors of high-speed spindle attachments used with computer-controlled cnc milling machines, wherein such high-speed spindle attachments allow sustained spindle speeds up to and including speeds above 20,000 rpm in milling machines otherwise capable without attachments of only achieving maximum sustained spindle speeds of approximately 5000 rpm or less. 
2. Description of the Related Art 
A high-speed spindle attachment for milling machines otherwise having a sustained maximum spindle speed of approximately 5,000 rpm, can be made compact and efficient by utilizing a high-speed electric motor controlled by an electronic controller designed to maintain a constant speed even under widely varying loads, such as described in U.S. Pat. No. 5,878,476 to Noelle (1999). One not readily apparent problem inherent in such systems, is the potential for  catastrophic damage to the spindle due to sudden power requirements beyond the level provided by the power supply and beyond the survivability of the motor to utilize. These sudden power requirements can occur unexpectedly when milling machine axes are under cnc control. The inherent problem stems from the fact that small high-power electric motors derive their power primarily from speed rather than torque. This speed/torque characteristic exposes the high-speed motors to overloads accruing from relatively moderate increases in torque demands. If the load imposed upon the motor exceeds the level for which it has been safely designed, the spindle will begin to slow down thereby increasing the chip load per tooth of the cutting tool. This further increases torque requirements in a regenerative fashion such that the process escalates quickly to the point that the spindle could stall completely while the machine axes are still moving, thereby causing major damage to the spindle before an operator would be able to realize what is happening and be able to slow the axes feed-rate. Other situations that may cause this problem include cnc grinding operations whereby a grinding wheel becomes clogged to the extent that rapidly accelerating torque demands occur. No known milling machine attachment for high-speed milling and grinding is known to include safeguards against this type of potential failure. 